The invention relates to a method for operating gas discharge lamps. Moreover, the invention relates to a ballast for operating gas discharge lamps.
During operation of a gas discharge lamp (also termed lamp below), the type of rooting of the discharge on the electrode depends on whether the electrode emits electrons (cathode) or captures them (anode). In the case of the anode, the discharge is rooted in a fashion distributed over a large area of the electrode, while in the case of the cathode a so-called focal spot (hot spot) is formed as a rule, as a result of which the discharge is rooted, rather, in a punctiform fashion. The point at which the focal spot is rooted depends on the electrode geometry, the electrode material and the temperature distribution on the electrode. These parameters are subject to changes during operation, such that the root point of the focal spot can change its position, and this is expressed by instability of the gas discharge (arc instability) or flickering. This flickering occurs, in particular, in the case of operation of the lamp with alternating current, since an electrode alternately forms a cathode and anode, and therefore the focal spot must reform with each change of the anode to the cathode.
So-called square-wave operation of the lamp is known, for example from U.S. Pat. No. 4,485,434, for the purpose of reducing flickering. It has emerged that it is advantageous to select a square-wave lamp current instead of a sinusoidal one for the stability of the AC operation of high-pressure gas discharge lamps. Customary values for the frequencies of the square wave are from 50 Hz to 200 Hz. Square-wave operation has become established in the case, in particular, of applications in image-recording and projection technology, where the constancy of the luminous flux is important. Commutation which is as fast as possible is aimed at in order for the time interval in which the luminous flux does not correspond to the square-wave amplitude to be as short as possible.
Despite the square-wave operation, the stability of the discharge is not yet satisfactory, in particular, in the case of short-arc high-pressure discharge lamps, which are preferred for use in projection technology. In order to improve the arc instability, PCT Application WO 95/35645 proposes a pulse-shaped rise in the lamp current at the end of a square wave period. The current rise is attended by a temperature rise which exerts a stabilizing influence on the position of the focal spot. Only approximate data are given on the duration and height of the pulses and on the operating frequency. Again, the mode or operation of the method is only indicated. Thus, the application of the method to a lamp of different design (for example with a different electrode geometry or different filling pressure) than the lamp addressed in the exemplary embodiment is possible only after extensive experimental work.
However, it is not only a problem to fix a suitable shape of the current curve but, as is set forth below, it is also a problem to produce a desired shape of curve. The load circuit of an arrangement for operating a discharge lamp includes, inter alia, energy stores which can also be parasitic, and the lamp, which constitutes a non-linear load.
The network of energy stores forms resonant frequencies which can be excited by the nonlinear load. Particularly in the case of the operation of short-arc high-pressure lamps, this leads to long-lasting transient phenomena after the commutation of the lamp current in the square-wave operation. These oscillations are also to be observed in the luminous flux, of course. In the case of applications which require high constancy of the luminous flux (e.g. video projection), it is therefore necessary to ensure that the time interval in which transient phenomena occur is short by comparison with the period of the square wave. The controller used in the relevant operating unit has a substantial influence on the duration of the transient phenomenon. A variable which constitutes a measure of the lamp power and is compared with a reference measure is produced in conventional operating units for the said applications. The result of this comparison supplies the manipulated variable for the power section of the operating unit. The settling time for a light source with square-wave operation can be defined by the time which elapses from the commutation up to the instant at which the luminous flux has adjusted itself in a band of +/xe2x88x925% about the setpoint. For the abovedescribed, conventional controller, this settling time is 250 xcexcs-300 xcexcs. Since the settling time should be at most 10% of a half period of the square wave, it follows that frequencies of at most 200 Hz can be realized for the square wave with conventional controllers.
According to the discussion on the prior art, the object of the present invention falls into two parts: firstly, the invention is intended to provide a method which permits virtually flicker-free operation of a gas discharge lamp with clearly defined parameters. Secondly, the invention is to provide means with the aid of which the above method can be implemented.
Secondly, in accordance with the preamble of claim 6 the invention is to provide means with the aid of which the above method can be implemented.
The first part of the object is achieved by means of a method having the characterizing features of claim 1. Particularly advantageous refinements are to be found in claims 2 to 5, which are dependent on claim 1.
As explained in the discussion on the prior art, the cause of the flickering of a lamp is based on the fact that the focal spot, which constitutes the root of the gas discharge on the cathode, changes its position continuously. A more precise analysis shows that no focal spot is formed directly after an electrode commutates to the cathode. Rather, what is firstly found is an area-wide discharge root. Only after a thermal inhomogeneity has been produced on the cathode does the discharge become constricted and form a focal spot. According to the invention, flickering of the lamp can be greatly reduced by carrying out commutation of the lamp current before the discharge forms a focal spot. Current edges which are steep with respect to time are required for an electrode to change as quickly as possible from cathode to anode, for which reason the method can be very effectively implemented by a square-wave current characteristic. Since a flicker-free operation is important, in particular, for applications in projection technology, the method is particularly important for lamps which are used in the case of such applications. These are chiefly high-pressure and extra-high-pressure discharge lamps and, because of the optical imaging qualities, particularly those having short discharge arcs. The frequency of the square-wave lamp current must be at least 300 Hz for such lamps, in order to satisfy the teaching of the method according to the invention.
If the method is applied for the first time to a specimen lamp, or if the lamp has mean time been operated using a different method, it is possible despite the application of the method according to the invention for flickering phenomena to occur for a short time after the lamp is taken into operation. The reason for this is an electrode structure which favors a quick formation of focal spots at different positions. The application of the method according to the invention, however, shapes the electrodes in such a way as to exert a stabilizing influence on the discharge arc. This produces a virtually flicker-free operation after a short time by means of the method according to the invention.
As described above, implementing the method according to the invention in the case of extra-high-pressure short-arc lamps requires a frequency of at least 300 Hz for the square-wave lamp current, while a frequency of at most 200 Hz can be implemented with operating units which include a conventional controller structure. The second part of the task of the present invention is to close this gap.
It is usual in an operating unit for gas discharge lamps to generate an output voltage UA from a constant, so-called intermediate circuit voltage U0 with the aid of a clocked DC/DC converter. Said output voltage is a DC voltage which can be set by a manipulated variable Us. The DC/DC converter can be of various types, such as, for example, step-up, step-down or inverse converters. With these converters, the manipulated variable Us varies the pulse duty factor of the circuit breakers included in the converters. The square-wave operation of the lamp is mostly implemented by virtue of the fact that the output voltage UA has its polarity reversed by means of a full bridge circuit with the desired frequency for the square wave.
The controlled variable of the operating unit is the power of the lamp (Pist). In cases where the lamp power can be determined only expensively, and the power loss of the operating unit is sufficiently accurately known, the input power of the DC/DC converter can also be used as a controlled variable. In conventional operating units, Pist is compared with a setpoint Psoll, and the manipulated variable Us is determined therefrom, without the assistance of further measured variables, directly or after weighting by a control characteristic proportional (P), proportional integral (PI), integral (I), proportional integral differential (PID)) However, no short settling time after commutation of the lamp current is possible by means of this structure.
According to the invention, the problem is solved by means of two measures: cascade control and feedforward control. Cascade control, as also applied in principle in the case of the so-called Current Mode in switched-mode power supplies, is implemented in the operating unit according to the invention by virtue of the fact that the weighted control difference from Pist and Psoll does not fix the value of the manipulated variable Us, but defines a setpoint for the lamp current Isoll. Isoll is compared with the value Iist, which constitutes a measure for the lamp current, and it is this result of comparison which first fixes the manipulated variable Us directly or after weighting by a control characteristic (P, PI, PID). The feedforward control is implemented as follows in the operating unit according to the invention: the output voltage UA, which is to be measured at the lamp terminals, is also a determining factor for the lamp power. Auxiliary circuits (for example ignition circuits) and supply means can lead to fluctuations in the output voltage UA. Fluctuations in UA interfere in the control process particularly in the case of the transient reaction after commutation of the lamp current. Consequently, according to the invention Isoll is determined not only by the control difference of Pist and Psoll, but is also brought into dependence on the output voltage UA. This can also be performed by means of weighting with a control characteristic, it being preferred to select a differentiating characteristic in order to accentuate the fluctuations in UA.
The invention is illustrated with the aid of the following figures.